A high-refractivity low-dispersion glass is in great demand as a material for optical elements such as various lenses. As a glass having such optical constants, a dense tantalum flint glass TaSF17 is known, which is described in “Glass Composition Hand Book” (Hiroshi Ogawa and Shin-ei Ogawa, issued by Japan Glass Product Industrial Society, 1991, page 106).
In recent years, with the rapid spread of digital cameras and video cameras, the demand for glass lenses as parts therefor continues to expand. On the other hand, with an increase in the number of pixels of imaging device in digital cameras, optical elements such as glass lenses are required to have high performances, and it is demanded to readily supply optical elements having high form accuracy.
As a method for highly productively supplying optical elements formed of a glass having high form accuracy, a precision press-molding method is known. However, conventional glasses such as TaSF17 and NbSF32 have high glass transition temperatures and are not suitable for precision press-molding.